The present invention generally relates to a process of making bonded multi-layer, flat-plate electrochemical cell devices, such as rechargeable batteries and supercapacitors. More specifically, the invention describes a process for establishing persistent interfacial bonding between laminated planar electrode and microporous separator members utilized in such electrochemical devices wherein such bonding is acheived at a low-temperature.
Widely deployed primary and secondary, rechargeable lithium-ion battery cells are typical of electrochemical devices to which the present invention is directed. Such cells comprise layers, or membranes, of respective positive and negative electrode compositions assembled with a coextensive interposed layer, or membrane, of electrically-insulating, ion-transmissive separator material. This multi-layer battery cell structure is normally packaged with a mobile-ion electrolyte composition, usually in fluid state and situated in part in the separator membrane, in order to ensure essential ionic conductivity between the electrode membranes during charge and discharge cycles of the battery cell.
One type of separator for this purpose is a microporous polyolefin membrane, either of single- or multi-layer structure, described, for example, in U.S. Pat. Nos. 5,565,281 and 5,667,911. When employed as rechargeable battery cell separators, these porous membranes not only effectively retain within their porous structure the essential fluid cell electrolyte compositions, but they also provide an additional advantage in that they possess an automatic cell "shut-down" feature that prevents uncontrolled heat buildup within the battery cell which might otherwise result, for instance during excessive cell recharging, in a dangerous explosive condition. This built-in safety mechanism occurs because the melting point range of the polyolefins utilized in the fabrication of the separator membranes is at the lower end of the danger zone of battery cell heat buildup. Thus, in the event of a run-away cell heating episode, the porous polyolefin separator membrane becomes heated to a point of melting and its pore structure collapses, thereby interrupting the essential ionic conductivity within the cell and terminating the electrochemical reaction before a dangerous condition ensues.
The packaging of battery cell structures has heretofore regularly taken the form of a metal "can", whether, for example, in elongated tubular or flattened prismatic shape, which has commonly been relied upon to not only contain the electrolyte component, but also to impart the significant stack pressure required to maintain close physical contact between the individual cell electrodes and the interposed separator member. This intimate contact, along with the composition of the electrolyte, is, as previously noted, essential to efficient ion transmission between electrodes during operation of the battery cell.
More recently, however, the profusion and continued miniaturization of electronic devices powered by Li-ion batteries and similar energy storage cells has generated a demand for a greater number of cell package shapes and dimensions, e.g., relatively broad, yet thin, lightweight packages having a significant degree of flexibility. For example, numerous end-use applications make thin, flexible envelope-style packages of polymer film more desirable than the prior rigid-walled high-pressure can containers. However, these more flexible packages are decreasingly capable of achieving and maintaining the substantial physical pressures required to ensure the noted essential intimate inter-layer contact throughout the battery cell.
In order to minimize the deleterious effect of degraded physical stack pressure previously relied upon to establish the necessary contact between cell layers, developers have progressed to the use of direct laminated adhesive bonding between electrode and separator layers to ensure their essential intimate contact. Typical of such innovations are battery cells utilizing polymer-based layer members, such as described in U.S. Pat. Nos. 5,456,000 and 5,460,904. In those fabrications, polymer compositions, preferably of poly(vinylidene fluoride) copolymers, which are compatible with efficient fluid electrolyte compositions are utilized in the physical matrix of both the electrode and the separator members to not only promote essential ionic conductivity, but also to provide a common composition component in those cell members which promotes strong interfacial adhesion between them within a reasonably low laminating temperature range. Such laminated, multilayer polymeric battery cells operate effectively with stable, high-capacity performance even though packaged in flexible, lightweight polymeric film enclosures.
Although such laminated battery cells, and like energy storage devices, have significantly advanced the art in miniaturized applications, the use of substantially non-porous polymeric matrices and membranes in their fabrication has deprived these devices of the desirable shut-down feature achieved when using the microporous polyolefin separator membranes. However, the high surface energy exhibited by the polyolefin membranes renders them highly abherent in nature and thus prevents their strong, permanent adhesion to electrode layer compositions, particularly within a reasonable temperature range which does not lead to melting or thermal collapse of the porous structure of the polyolefin membranes.
Some attempts have been made by electrochemical cell fabricators to combine, by simple solution overcoating or extrusion, the shut-down properties of porous separator membranes with the laminate adhesive properties of polymer compositions, for example, as described in U.S. Pat. Nos. 5,837,015 and 5,853,916. However, it has generally been found that the overcoating compositions significantly occlude or otherwise interfere with the porous structure of the polyolefin membranes and cause a deleterious decrease in electrolyte mobility and ionic conductivity. Further, the addition of substantial amounts of overcoating materials, increases the proportion of non-reactive components in a cell, thereby detracting from the specific capacity of any resulting energy storage device.
As an alternative approach to enabling the incorporation of microporous separator membranes into a laminated electrochemical cell structure, an attempt to modify the surface of the polyolefin membrane by application of a minimal layer of polymer composition has been made. The polymer composition would not be of such excessive thickness as to occlude the porosity of the membrane, but rather would provide an intermediate transition in compatibility to the matrix polymer of preferred electrode cell layer compositions. Thus, for example, a thin layer from a dilute solution of poly(vinylidene fluoride) copolymer is applied to the microporous separator membrane when the membrane is intended to be employed in the fabrication of a battery cell by thermal lamination with electrodes comprising active compositions of a similar polymer. This modification has proven to be insufficient in itself to enable satisfactory interfacial bonding between cell component layers at lamination temperatures below the critical level which results in collapse of separator porosity and its attendant loss of effective ionic conductivity and desirable shut-down capability.
Therefore, there remains a need in the art to provide improved methods of bonding high-capacity, shut-down protected, electrochemical cells through the use of surface-modified microporous separator membranes. There also remains a need in the art for improved methods for effectuating the use of surface-modified microporous separator membranes in high-capacity, shut-down protected laminated electrochemical cells.